refactor(pack): host custom jars from ./custom outside the pack root

Move pack/custom/ -> ./custom/ and bind-mount it into caddy at
/srv/pack/custom (served URL pack.${BASE_DOMAIN}/custom/ unchanged).
Keeping jars outside the packwiz pack root stops `packwiz refresh`
from indexing them as direct files on top of the mods/*.pw.toml
external refs (drops 9 bogus custom/*.jar entries from index.toml).

add-custom-mod.sh now writes jars to ./custom/. Trim the blessingskin
caddy override to just the conf swap (volumes merge by target).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -31,6 +31,10 @@ services:
       - ./mirror/launcher:/srv/launcher:ro
       - ./caddy/caddy-root-ca.crt:/srv/ca-pub/ca.crt:ro
       - ./pack:/srv/pack:ro
+      # Custom (locally-hosted) mod jars live OUTSIDE ./pack so packwiz refresh
+      # doesn't index them as direct files — they're external download targets
+      # referenced by pack/mods/*.pw.toml. Served at pack.${BASE_DOMAIN}/custom/.
+      - ./custom:/srv/pack/custom:ro
     networks:
       mcnet:
         # Only the stack's own subdomains in the base. The upstream spoof-host